PAP (pokeweed antiviral protein) is known to retard virus replication by enzymatically inhibiting protein synthesis on eukaryotic ribosomes. The effects of its action are very similar to those of the A chain of the anti-tumor enzyme ricin also under study in our laboratory. The enzymes attack the 60S subunit of the ribosomes but the nature of the catalyzed reaction is totally unknown. In fact it is not even clear whether they recognize ribosomal proteins, RNA or both. We have crystallized PAP in a form suitable for high resolution x-ray analysis and have obtained three isomorphous heavy atom derivatives of the protein. A 4.5 angstrom units A map has been produced and shows the enzyme to be composed of two roughly cylindrical domains, each of roughly 12 to 15 thousand daltons. We propose to carry this structural analysis to 2.8 angstrom units A resolution using the derivatives presently available and to build a molecular model of the toxin. We hope to compare the structure with ricin, for which a 2.8 angstrom units A map is presently available.